This type of reactor includes a core, disposed at the center of a tank, the upper part of the latter being closed by a slab on which the tank is suspended. The core is immersed in liquid sodium used as the coolant fluid. A sodium-compatible blanket gas, usually argon, occupies the free space of the tank between the liquid sodium level and the slab. The blanket gas constitutes a radioactive environment, at a high temperature, and containing sodium aerosols.
After a shutdown of the reactor, in particular to carry out normal operations for maintenance of the fuel assemblies, the temperature of the blanket gas is reduced to approximately 150.degree.. This temperature corresponds to that which those skilled in the art call a cold shutdown of the reactor.
During these shutdowns, the normal maintenance operations are carried out by using means adapted to maintenance of the fuel assemblies. Outside these normal operations, it is possible to carry out exceptional work in the radioactive parts of the reactor, for example disposing a video camera in the blanket gas, which necessitates the work of a robot or of a jointed remote manipulation arm which can move analogously to a human arm. The type of remote manipulation arm, for which the device which is the subject of the invention is designed, can operate normally in radioactive media but it is in general unfit for high temperatures, in particular for the temperature of the blanket gas at the time of a cold shutdown of the reactor.